The castle on Lochindorb is a little secret hidden away from the main tourist areas. What a great photo so we now know what size it really is - from the loch it just looks likes a pile of stones!
Marie (Scotland Made Easy)

If you go to the lodge on the shore, the ghillie there will hire you a rowing boat, and you can row over to take a look at it. You are no longer supposed to land on the island, as some of the structure is considered unsafe ("Elf & safety"!)but that won't prevent your having a closeup look at the whole thing.